Due to improvements in their durability and performance Ventricular Assist Devices (VADs) have increasingly been used as a treatment of heart disease. In the past VADs traditionally featured displacement pumps which provided pulsatile support to the failing heart. Recently, rotary blood pumps have been used in VADs, due to the smaller size, higher efficiency and durability when compared with pulsatile pumps. Current commercially available VADs utilize both centrifugal and axial flow type rotary pumps to provide augmented perfusion to the circulatory. One of the lingering challenges with rotary VADs is their interaction with the circulatory system and native heart. Operation of the rotary VAD at a low speed could cause underperfusion of the circulatory system or regurgitate flow through the VAD and conversely an excessively high speed could cause a collapse of vessels in the inflow path potentially resulting in a suction event. Accordingly a correct operation of the device is mandatory and will improve patient quality of life and outcomes.
Rotary VADs are typically set at a constant operational speed by a clinician or trained operator. A change to the patient condition, either can alter the interaction between the device and the circulatory system. In the case of a change in patient condition, the previously set rotation speed of the VAD may no longer provide the desired level of cardiac support. Monitoring patients with these devices and their physiological changes is a difficult task. There are many methods of patient evaluation, such as echocardiaography, catheter based pressure and volume measurements as well as intrinsic feedback from the device itself. However most of these methods require a skilled practitioner to perform the evaluation or are invasive.
A noninvasive physiological monitoring and control system for a rotary blood pump would provide an indication of the patient condition for a particular VAD speed and determine a speed range to operate the device to not to only prevent undesirable events, but also to provide an improved level of support for the patient to promote recovery.
According to the well known state of the art currently the most common form of rotary blood pump control is fixed speed operation. A trained clinician or technician sets a target speed for the device to operate at and the device will attempt to operate at or close to this set speed value.
One method for determining an appropriate VAD speed is to find the VAD speed such that the Aortic valve is closed throughout the cardiac cycle. The speed at which the Aortic valve closes throughout the cardiac cycle is called the Aortic Valve Closure Speed (AVCS). Determining the AVCS can be determined using echocardiography imaging while the pump speed is increased. The AVCS is dependent on the heart's contractility level, preload, afterload and importantly the support of VAD. Accordingly it may change over time. The status of the Aortic valve is a critical piece of information for clinicians when assessing the condition of a patient with a VAD.
In the case of a VAD operating under constant speed control, any change, either short term or long term, to the patient physiological state will not elicit a change in the VAD's operational speed. To improve on the method of fixed speed control, feedback from the physiological system or device has been used to indicate both changes to the physiological system and a potentially appropriate VAD operation point.
Many of physiological feedback techniques utilize sensorless, or intrinsic measurements derived from the device's motor signals. Methods of estimating the pump head pressure, pump flow, heart beat and the pulsatility of the native heart from signals from the device have been proposed by a number of authors and have been implemented in commercially available pumps, for example according to U.S. Pat. No. 7,887,479, US Patent Application 20110054239. This feedback information is typically displayed to the clinical staff to assist in their diagnosis of the pump state.
Other feedback mechanisms that utilize intrinsic measurements seek to determine the status of the native heart through the analysis of pump signals. One such method is the pulsatility index which is a measure of the pulsatility of the native heart when operating with a VAD. The pulsatility of a signal such as motor power or current can give an indication of the pulsatility of the ventricle, and therefore the assistance of the device.
Pressure sensors placed at the inlet or outlet of the VAD or cannulas have been suggested to determine the afterload, preload or pressure difference over the device, however the long term accuracy and stability of pressure sensors has restricted this technique's implementation in long term implantable devices. Sensor based feedback based on a flow sensor placed on the outlet cannula, has been included on a commercially available device.
Due to the difficulties in regularly monitoring and assessing changes to the circulatory hemodynamics the speed of the VAD is typically set by the clinician or trained expert and remains fixed until the next evaluation is performed by the specialist.
A monitoring system that could evaluate the patient's physiological status, such as if the Aortic valve is closed, would be invaluable to the specialists to determine if the VAD speed needs to be reviewed.
In cases when patient recovery is expected, the status of the Aortic valve is critical for evaluating the recovery. Weaning a patient off a VAD is a very difficult procedure where the native heart function must be evaluated to determine if the VAD could be removed from the patient. One of the best indications of the patient's left ventricle contractile state is the AVCS. An increase of contractility (patient native heart improvement) will see an increase in the AVCS while a decrease in contractility will see the AVCS fall.
Many authors have described the risk of consistently operating a VAD at a speed at which the Aortic valve does not open during the cardiac cycle. Long-term operation in this condition can lead to fusion of the leaflets and degrading the functionality of the valve. As such it is important to be able to periodically ensure that the pump operates, even if only temporarily, at a speed at which the valve leaflets open. The most robust method for determining the status of the Aortic valve is to use
Echocardiography, which must be performed by a trained technician or clinician and as such is often only performed when required or scheduled. As such, due to the difficulties in assessing the operation of the aortic valve, any changes to the AVCS is not monitored regularly.
Currently many of the controllers and feedback measurements are based around sensorless or intrinsic measurements. Although these sensorless methods can provide some indication of the current status of the pump, however they fail to give adequate information regarding the physiological state of the heart or circulatory system. Many of these measurements of pump performance inferred from motor or device status can be susceptible to parameter variations including blood viscosity, blood clots and temperature.
Other derived measurements such as pulsatility index can be susceptible to short term changes in contractility or preload making them inappropriate for tight physiological monitoring. Sensor based feedback can provide more direct estimation of the pump operation, however drift of the sensors over their long implantable lifetime has made their clinical use limited.